This invention relates to a base for low pressure discharge lamps, and, more particularly, to a base structure which allows the base to be fixed to the lamp envelope without the need for basing cement.
The base of a low pressure discharge lamp provides for the electrical and mechanical connection of the lamp to a socket. Generally, a lamp base comprises a metallic or plastic base shell fitted to an end portion of the lamp envelope and contact elements responsible for electrically and mechanically connecting the lamp to the socket. In the event that the base shell is made of metal, an end portion of the base shell is closed by an insulator disc which embeds one or two base studs to which the lamp lead wires are electrically secured.
U.S. Pat. No. 4,221,453 discloses a low pressure discharge tube which has a base made of elastic, bendable, thermoplastic material. The base shell has an annular wall with knobs protruding inwardly. When the base shell is fitted to the tube, the annular wall elastically deforms and the knobs snap into a groove formed in an end portion of the tube. The base shell has also a frontal portion which closes the tube. This base structure suffers form several drawbacks. One drawback is that the material of which the whole base shell is made has bad thermal conductivity. A linear low pressure discharge tube, in which the additive in the gas fill is liquid mercury, has a cold spot in the vicinity of the base. An appropriate quantity of mercury is condensed in the cold spot in order to ensure an optimum mercury vapor partial pressure in the tube. Owing to the bad thermal conductivity of the base material, the base will not cool the cold spot sufficiently. Another drawback is that the base made of thermoplastic material may deform at unfavorable circumstances due to the heat generated by the lamp during its operation. A further draw back is that the base made wholly of thermoplastic material is susceptible to aging and inflammable.
U.S. Pat. No. 5,432,400 describes a lamp base structure which allows the lamp base to be retained on the end portion of the lamp without the need for basing cement. The lamp base includes a metallic base shell which has an annular flange. This flange is heated prior to fitting over the end portion of the lamp envelope. Cooling of the flange after fitting reduces the flange diameter thereby providing an interference fit with the end portion.
This base structure represents a simple and straightforward solution to the problem of fixing the base to the lamp envelope without basing cement however has drawbacks, as well. In the case of low pressure discharge lamps with small diameter tube, the surface along which the interference fitting comes into being is also small and the lamp base can turn around the end portion on the effect of relatively small torque. Standards require that the lamp base must withstand a torque of 1. 5 Nm, and this requirement cannot be securely met at tube diameters smaller then one inch. An appropriate cooling of the cold spot of the lamp can hardly be accomplished since the annular flange of the base shell contacts with the glass tube on a relatively small surface. This can lead to a reduction in lumen output of the lamp. At low pressure discharge tubes generally, a stem mounted with electrode is melted into the end portion of the tube to form a sealing region, most often a seal region portion. Owing to this manufacturing process, dimensions of the sealing region have a great variation which leads to a similarly great variation in the tightening force arising between the lamp and the lamp base as well as in the surface conducting the heat away from the cold spot. When the base shell with the annular flange is snapped onto the seal region portion of the glass tube, the glass can easily be got damaged. Last but not least, the manufacturing process of this base structure needs special equipment which properly heats up the flange of the metallic base shell. This makes the manufacturing process relatively difficult.
Thus there is a particular need for a base structure without base cement which provides sufficiently great tightening force with small variation between the glass tube and the base even in the event of small diameter tubes, provides for an appropriate cooling of the end portion of the tube, and does not require special equipment for its manufacturing.
As an exemplary embodiment of the present invention, a lamp base in combination with a low pressure discharge lamp is provided. The low pressure discharge lamp comprises a glass envelope which has an end portion and an axis. A stem is melted into the end portion of the envelope in a gas-tight manner in order to form a seal region. The stem is mounted with an electrode and includes a current in-lead which is connected to the electrode. The lamp base comprises a metallic base shell which is fitted to the seal region. A resilient insert is disposed substantially in the base shell. Said resilient insert embraces the seal region at least partly, and fits resiliently to an interior surface of the base shell. Said insert has means suitable for positioning the lamp base in cooperation with further means formed on the surface of the seal region. The base shell has an end portion closed al least partly by an insulator piece which embeds a base stud made of metal. The base stud is connected to the current in-lead.
The proposed base structure has a number of advantages over the prior art represented by the patents cited above. One advantage is that this structure provides a sufficient tightening force between the end portion of the envelope of the lamp and the base shell due to the resilient insert fitting to the inner surface of the base shell and connecting with the seal region of the envelope through the positioning means. This tightening force is practically independent from the size of the envelope. Another advantage is that this base structure provides appropriate cooling for the cold spot in the envelope due to the metallic base shell which contacts with the glass envelope on a relatively large surface primarily through the resilient insert. A further advantage over U.S. Pat. No. 5,432,400 is that the manufacturing process of this base structure does not require a step which is unusual in lamp basing and needs special equipment. A still further advantage is that the proposed base structure is durable and is not apt to take fire.